For koshi nage, we are being taught to bring our feet together when doing the technique. However, Saito sensei is doing his with an open stance.

It looks easier with Saito sensei's version since when you are face to face with your uke, you just step straight toward him and do a 90 degree hantai tenkan. And with an open stance, your back is automatically bent before throwing so its more economical. No need to bend down after positioning yourself to uke unlike with a feet together stance which is an extra process.